This application claims the benefit of International application PCT/JP99/07225 and Japanese Patent application Nos. 10-370143 and 11-066769 which are hereby incorporated by reference.
The present invention relates to a catadioptric imaging system which is preferably used in projection exposure for producing, for example, a semiconductor device, a liquid crystal display device, or the like, by photolithography, as well as a projection exposure apparatus and an exposure method using such catadioptric imaging system, and more particularly, to a catadioptric imaging system, or the like, which attains a resolution of 0.1 xcexcm or lower in the ultraviolet region by using a reflection system as a factor of an imaging optical system inside a catadioptric imaging system.
In a process of photolithography for producing a semiconductor device, or the like, there is employed a projection exposure apparatus which performs projection exposure of a pattern image formed on a photo mask or a reticle (hereinafter collectively called the xe2x80x9creticlexe2x80x9d) on a wafer or a glass plate with photo resist coated thereon through a projection optical system. Then, with enhancement of the degree of integration of the semiconductor device, or the like, a resolving power required for the projection optical system used in the projection exposure apparatus is gradually increasing. To satisfy this requirement, it is needed to reduce the wavelength of illumination radiation (exposure light) and to enlarge the numerical aperture (NA) of the projection optical system. For example, in case that the wavelength of the illumination radiation is not more than 180 nm, a high resolution of 0.1 xcexcm or less can be achieved.
However, if the wavelength of the illumination radiation is reduced, light absorption increases and the kinds of practicable glass materials are limited. Particularly, if the wavelength becomes 180 nm or less, only fluorite is practicable as the glass material. For this reason, in a projection optical system which is constituted only by a refractive lens system, or only by lens components containing no reflecting mirror with a refracting power (a concave reflecting mirror or a convex reflecting mirror), it becomes impossible to correct a chromatic aberration.
Also, since an optical performance required for a projection optical system is extremely high, it is required to correct the aberrations to be substantially zero. However, in order to achieve a desired optical performance in a dioptric projection system, a large number of lens components is required, so that it is inevitable to reduce the transmittance or to increase the production cost.
In contrast, a catadioptric system using a power (the refracting power) of a concave reflecting mirror, or the like, that is, an optical system which contains a reflecting mirror having a refracting power without containing a lens component generates no chromatic aberration and shows a contribution having a sign reverse to that of a lens component with respect to the Petzval sum. Accordingly, in an optical system which is a combination of a catoptric system and a dioptric system, or a so-called optical system of catadioptric type (hereinafter called the xe2x80x9ccatadioptric imaging systemxe2x80x9d), various aberrations including a chromatic aberration can be satisfactorily corrected to be substantially zero without increasing the number of lenses. In this case, the catadioptric imaging system is an optical system which contains at least one lens component and at least one reflecting mirror having a refracting power. In this respect, there is no need to say that a plane parallel plate or a plane reflecting mirror for deflecting an optical path may be provided if needed.
However, if a concave reflecting mirror is used in an optical path of a projection optical system of a projection exposure apparatus, light incident on this concave reflecting mirror from the reticle side is reflected to move back to the reticle side again. For this reason, there are conventionally proposed various technologies for separating an optical path for the light incident on the concave reflecting mirror from an optical path for the light reflected by the concave reflecting mirror so as to lead the reflected light from the concave mirror toward the wafer, that is, the technologies for constituting a projection optical system by a catadioptric imaging system.
As a representative method for separating optical paths from each other, a method for separating optical paths from each other by using a transmission reflecting surf ace such as a half mirror or a polarizing beam splitter is proposed in Japanese Patent Publication No. HEI 7-117648. Also, in U.S. Pat. No. 4,779,966, there is proposed a method for separating optical paths from each other by forming an intermediate image by using an off-axis optical path and providing a plane mirror for bending optical paths in the vicinity of the forming position of the intermediate image. Further, in U.S. Pat. No. 5,031,976, there is proposed a method for separating optical paths from each other by using two reflecting mirrors each having an opening at the center thereof, and providing the two reflecting mirrors so that a beam is reflected when the section of the beam is large in the vicinity of the pupil of the optical system and the beam passes through the central openings when the section of the beam is small in the vicinity of the image plane.
However, since the optical path separation methods disclosed in Japanese Patent Publication No. HEI 7-117648 and U.S. Pat. No. 4,779,966 employ-a plane mirror which is provided in an inclined manner with respect to the optical axis for the optical path separation, an optical system is required to have a plurality of optical axes. In a projection optical system which requires adjustment of the optical components with high precision, a highly sophisticated technology is required for positioning the plurality of optical axes with precision and to dispose the optical components at desired positions with respect to the respective optical axes in the order of microns. As a result, it is inevitable to increase the cost for producing the optical system.
On the other hand, according to the optical path separation method disclosed in U.S. Pat. No. 5,031,976, all of the optical elements for constituting the optical system can be disposed along the single optical axis. As a result, it is possible to produce the optical system with precision in accordance with the adjusting method of the optical components which is conventionally used in a projection optical system. Optical systems employing such optical path separation method are disclosed in U.S. Pat. Nos. 5,488,229, 5,650,877, 5,717,518, and the like, in addition to U.S. Pat. No. 5,031,976.
However, in the optical system disclosed in U.S. Pat. No. 5,031,976, or the like, it is required to shield a part of the beams centering the optical axis, out of imaging beams, in order to prevent stray light from being generated, which passes, without being reflected by two reflecting mirrors at all, through the central openings thereof to reach the image plane. As a result, due to this central shielding of the imaging beams, the image forming characteristic of the optical system is degraded. Accordingly, in order to apply the optical path separation method disclosed in U.S. Pat. No. 5,031,976 to a projection optical system, it is essential to suppress the rate of central shielding of the imaging beams (hereinafter simply called the xe2x80x9ccentral shielding ratexe2x80x9d) to the minimum, so as to obtain a sufficient optical characteristic.
In the optical system disclosed in U.S. Pat. No. 5,650,877, a half mirror is arranged to be close to an object plane (a plane corresponding to a mask plane) while a reflecting mirror having an opening at the center thereof is arranged to be close to an image plane (a plane corresponding to a wafer plane), without forming an intermediate image. In this manner, the central shielding rate is suppressed to some extent. That is, in this optical system, it is inevitable to employ a half mirror. However, when this optical system is applied to a projection optical system which uses exposure light having the wavelength of 180 nm or less, materials usable for forming a half-transmitting thin film are limited, so that it is difficult to produce a half mirror having a satisfactory performance. Also, only not more than a fourth of the light quantity reaches the wafer plane, so that the throughput is inevitably lowered.
In either of the optical systems disclosed in U.S. Pat. Nos. 5,031,976, 5,488,229, and 5,717,518, an intermediate image is formed through a first imaging optical system, a first reflecting mirror having a central opening is provided in the vicinity of the forming position of the intermediate image, and further a second reflecting mirror having a central opening is provided in the vicinity of the image plane. Thus, the central shielding rate is suppressed to some extent.
However, these disclosed optical systems have the drawbacks as described below.
That is, in order to obtain a high resolution of not more than 0.1 xcexcm, a projection optical system generally requires an NA of not less than 0.7 on the image side even when using a F2 laser (with the wavelength of 157 nm) as exposure light. Also, taking the size of a semiconductor chip and the throughput into account, an image circle on the image side with the diameter of less than 10 mm can not be considered currently. Further, it is also impossible to largely reduce a WD on the image side (a working distance which is an axial air space between the lens surface which is closest to a wafer and the wafer in the projection optical system) on the image side, taking into account an influence of a gas out from a resist (which is coated on the wafer) at the time of exposure and an influence of a drive of a wafer stage.
In the optical system disclosed in U.S. Pat. No. 5,650,877, a thick lens is provided in the vicinity of the forming position of the intermediate image and another thick lens is employed in a refracting portion of a rear surface reflecting mirror serving as a second reflecting mirror, so that a chromatic aberration generated in the first imaging optical system is corrected by using the chromatic aberration correction by a thick lens which is conventionally known. However, in this optical system, in order to satisfy the above requirements required for a projection optical system (for the image-side NA, the image circle diameter, the image-side WD, etc.), it is inevitable to increase the thickness of the refracting portion of the rear side reflecting mirror and to increase the diameter of the rear surface reflecting mirror conspicuously in proportion to the thickness of this refracting portion. As a result, not only the production of such optical system becomes difficult, the manufacturing cost increases drastically.
Moreover, in this optical system the object side (the image side) is assumed to be approximately infinity, so that the refracting power of the first imaging optical system is small, and chromatic aberration in the first image optical system is not generated in a large amount, thereby attaining a chromatic aberration correction by the thick lens which is satisfactory to some extent. However, when such optical system is applied to a projection optical system in which a reduction rate of 0.15 to 0.4 or around is required for the entire system, the refracting power of the first imaging optical system inevitably increases, and the chromatic aberration in the first imaging optical system generated in the first imaging optical system also inevitably increases. As a result, in this optical system, it is difficult to correct the chromatic aberration satisfactorily while maintaining particularly the diameters and the thickness of the two reflecting mirrors to be practicable.
The optical system disclosed in U.S. Pat. No. 5,488,229 is a variation of the optical system disclosed in U.S. Pat. No. 5,031,976. This optical system is obtained by assuming an optical system for a laser repair apparatus (an apparatus for repairing a semiconductor circuit by laser processing) using an ArF excimer laser (having an oscillating wavelength of 193 nm) and optimizing it. In this optical system, a chromatic aberration is corrected without using a thick lens, but using two rear surface reflecting mirrors. However, a concave mirror which is provided in the vicinity of the forming position of the intermediate image is a rear surface reflecting mirror for the purpose of correcting the chromatic aberration. Thus, the diameter of the concave mirror is conspicuously large for attaining the image-side NA of not less than 0.7 and for attaining an image circle in the required size on the image side, so that this system can not be a practicable optical system as a projection optical system for a semiconductor exposure apparatus, or the like.
In the optical system disclosed in U.S. Pat. No. 5,717,518, the image-side NA of 0.8 Is achieved. However, in this optical system, like in the optical system disclosed in U.S. Pat. No. 5,488,299, a concave mirror which is provided in the vicinity of the forming position of an intermediate image is a rear surface reflecting mirror for the purpose of correcting the chromatic aberration, so that the diameter of the concave mirror becomes conspicuously large and not practicable to achieve an image circle having a predetermined size on the image side. Also, according to this conventional technology, an optical system is constituted by a plurality of glass materials such as quartz glass or fluorite, in order to correct a chromatic aberration satisfactorily. For this reason, in this optical system, exposure light having the wavelength of 180 nm or less can not be used, and the ArF excimer laser light is light of a practicable shortest wavelength.
In an optical system employing exposure light having the wavelength of 180 nm or less fluorite is desirably used for a refracting member in order to obtain a sufficient refractive index. However, when a fluorite lens with a rear surface reflecting coat applied thereon is used, the following drawbacks may be generated. That is, film material for the coat for reflecting light in such wavelength range are limited and light absorption thereof is comparatively large. Thus, a large light energy absorbed by the coat is transmitted to the fluorite lens as heat. As a result, such tendency is liable to occur that the lens surface shape of fluorite having a large coefficient of thermal expansion varies during exposure so as to deteriorate the image forming performance.
The present invention is contrived taking the above drawbacks into consideration, and the object of the invention is to provide a catadioptric imaging optical system which can obtain an image-side NA and an image circle having predetermined sizes with a small number of lenses and without enlarging a reflecting mirror to attain a high resolution of, for example, 0.1 xcexcm or less, even when an illumination radiation in the ultraviolet range having the wavelength of, for example, 180 nm or less is employed, as well as a projection exposure apparatus, or the like, provided with such an optical system.
A catadioptric imaging system according to a first aspect of the invention is provided with a first imaging optical system of a dioptric type for forming a primary image on a first plane and a second imaging optical system of a catadioptric type for forming a secondary image of said first plane on a second plane with reduction magnification on the basis of the light from said primary image. In this catadioptric imaging system, the first imaging optical system comprises a first lens group having a positive refracting power, an aperture stop, and a second lens group having a positive refracting power in this order from the first plane side. The second image optical system comprises a primary mirror having a front surface reflecting surface in a concave form and a first radiation transmitting portion at the center thereof, a secondary mirror having a second radiation transmitting portion at the center thereof, and a lens component provided adjacently to the secondary mirror on the primary mirror side and having a negative refracting power. The light from the primary image passes through the first radiation transmitting portion of the primary mirror and the lens component and is reflected by the secondary mirror, the light reflected by the secondary mirror passes through the lens component and is reflected by the primary mirror, and the light reflected by the primary mirror passes through the lens component and the second radiation transmitting portion of the secondary mirror to form the secondary image on the second plane, and all the refracting optical members for constituting the catadioptric imaging system are formed of optical materials having the same index of refraction.
In the above catadioptric imaging system, the primary image of the first plane (the object plane), that is, an intermediate image, is formed through the first imaging optical system. Then, in the vicinity of the forming position of the intermediate image, there is provided the primary mirror having the front surface reflecting surface in a concave form and having the first radiation transmitting portion (the central opening) at the center thereof, and the diameter of this central opening of this primary mirror is suppressed to reduce the central shielding rate, thereby avoiding deterioration of the image forming performance. Further, such arrangement is employed in which in the vicinity of the second plane (the image plane), there are provided the secondary mirror with a reflecting surface having the second radiation transmitting portion (the central opening) at the center thereof and the lens component adjacently to the secondary mirror on its primary mirror side, so that the central opening of this secondary mirror can be approximated to the second plane. Thus, the central shielding rate is reduced to avoid the deterioration of the image forming performance.
As stated above, when the optical system of the present invention is applied to a projection optical system of a projection exposure apparatus, it is required to use an exposure light having the wavelength of 180 nm or less in order to obtain a high resolution of 0.1 xcexcm or less. Thus, according to the first aspect of the invention, all the refracting optical members for constituting the catadioptric imaging system are formed of optical materials having the same index of refraction, including, for example, a single glass material which is capable of transmitting an F2 laser beam with a sufficient transmittance. Moreover, it is possible to perform a primary chromatic aberration correction by making the refracting power of the lens component provided adjacently to the secondary mirror on its primary mirror side to be negative.
The catadioptric imaging system according to a second aspect of the invention is provided with a first imaging optical system of a dioptric type for forming a primary image of a first plane and a second imaging optical system of a catadioptric type for forming a secondary image of said first plane on a second plane with reduction magnification on the basis of the light from said primary image. The first imaging optical system comprises a first lens group having a positive refracting power, an aperture stop, and a second lens group having a positive refracting power in this order from the first plane side. The second image optical system comprises a primary mirror having a front surface reflecting surface in a concave form and a first radiation transmitting portion at the center thereof, a secondary mirror having a second radiation transmitting portion at the center thereof, and a lens component provided adjacently to the secondary mirror on the primary mirror side. The light from the primary image passes through the first radiation transmitting portion of the primary mirror and the lens component and is reflected by the secondary mirror, the light reflected by the secondary mirror passes through the lens component and is reflected by the primary mirror, and the light reflected by the primary mirror passes through the lens component and the second radiation transmitting portion of the secondary mirror to form the secondary image on the second plane, and at least one out of all the refracting surfaces and the reflecting surfaces for constituting the catadioptric imaging system is formed to be aspherical.
In the above catadioptric imaging system, at least one of all the refracting surfaces and the reflecting surfaces for constituting the catadioptric imaging system is formed to be aspherical. With this arrangement, it is possible to achieve an optical system having a sufficient image forming performance while suppressing the sizes of the primary mirror and the secondary mirror to be practicable.
According to the catadioptric imaging system of the first or second aspect of the invention described above, even when a light having the wavelength of 180 nm or less such as an F2 laser beam is used, the size of the primary mirror is not particularly increased and an image-side NA and an image circle in the predetermined sizes can be securely obtained, so that it is possible to realize a catadioptric imaging system capable of obtaining a high resolution of, for example, 0.1 xcexcm or less by using a small number of lenses.
In this respect, in the optical systems of the present invention (according to the first aspect or the second aspect), it is preferable to dispose the reflecting surface of the secondary mirror on the refracting surface of the lens component which is provided adjacently to the secondary mirror so as to compose the rear surface reflecting mirror of the secondary mirror and the lens component. With this structure, it is possible to approximate the secondary mirror, and consequently the central opening thereof, to the second plane further, without protruding a holding mechanism of the secondary mirror to the second plane side. As a result, it is possible to further reduce the diameter of the central opening of the secondary mirror, and consequently the central shielding rate, so as to avoid deterioration of the image forming performance more excellently.
Also, according to the optical system of the present invention, it is preferable to form the reflecting surface of the secondary mirror to have the concave surface on the primary mirror side. With this structure, it is possible to further suppress the central shielding rate without increasing the diameter of the primary mirror.
Also, according to the optical system of the present invention, it is preferable to form the refracting surface on the primary mirror side of the lens component which is provided adjacently to the secondary mirror to have the concave surface on the primary mirror side. With this structure, it is possible to achieve excellent chromatic aberration correction.
In order to achieve excellent chromatic aberration correction, it is preferable to satisfy the following conditional expression (1):
0.03 less than D/|R| less than 1.0xe2x80x83xe2x80x83(1). 
Here, R represents the radius of curvature of the refracting surface on the primary mirror side of the lens component provided adjacently to the secondary mirror. Also, D represents the clear aperture diameter of the secondary mirror.
Above the upper limit of the conditional expression (1), a coma aberration and high-order aberrations of a spherical aberration are unfavorably generated to prevent a large numerical aperture from being attained. On the other hand, below the lower limit of the conditional expression (1), it becomes unfavorably impossible to correct the chromatic aberration with excellency while maintaining the secondary mirror to be in a realizable size. Note that it becomes possible to correct the chromatic aberration and the other aberrations at the same time with more excellency by setting the upper limit in the conditional expression (1) to be 0.5 and the lower limit to be 0.07.
Incidentally, when the wavelength of the exposure light becomes 180 nm or lower, film materials for the antireflection coat to be applied on the surface of the lens component are limited, and a sufficient performance can not be obtained as the antireflection coat. As a result, it is required to minimize the number of transmitting surfaces such as lens surfaces. Accordingly, in the present invention, it is preferable that the lens component provided adjacently to the secondary mirror is the only refracting optical member to be provided in an optical path between the primary mirror and the secondary mirror. With this arrangement, it is possible to reduce the number of lenses (consequently the number of transmitting surfaces) so as to enhance the transmittance of the optical system. However, this arrangement does not prevent a plane parallel plate or the like from being provided in an optical path between the secondary mirror and the second plane.
Also, according to the optical system of the present invention, at least one of the refracting surface and the reflecting surface of the second imaging optical system is formed to be aspherical, so that it becomes possible to prevent the primary mirror and the secondary mirror from being enlarged and the central shielding rate from increasing. As a result, it is possible to provide an optical system which has a sufficient image forming performance and is practicable.
According to the optical system of the present invention, it is possible to reduce the number of the lenses while attaining a high resolution of 0.1 xcexcm or lower to further enhance the transmittance of the optical system, by forming at least one of the refracting surfaces in the first imaging optical system to be aspherical.
Further, according to the optical system of the second aspect of the invention, it is preferable that at least one of the refracting surface and the reflecting surface in the second imaging optical system is formed to be aspherical and each of the first and second lens groups has at least one aspherical refracting surface. With this arrangement, the number of the lenses for constituting the catadioptric imaging system can be reduced, and a light having the wavelength of 180 nm or less can be used as the exposure light.
Also, according to the optical system of the present invention, it is preferable that at least 80 percents of all the lenses for constituting the catadioptric imaging system are one sided aspherical lenses, in which one of the refracting surfaces is formed to be aspherical and the other to be spherical. With this arrangement, it is possible to realize an ideal optical system which has a high resolution of 0.1 xcexcm or lower, taking the size of the optical system, the image forming performance, transmittance, and an aberration fluctuation depending on illumination heat, etc., into consideration. It is also possible to adjust the eccentricity from the aspherical lens surface in reference to the spherical lens surface easily by employing the so-called one sided aspherical lens.
Further, it becomes possible to use an exposure light having the wavelength of 180 nm or lower, such as an F2 laser beam (157 nm) to thereby realize a high resolution of 0.1 xcexcm or lower, by forming all of the refracting optical members for constituting the catadioptric imaging system of fluorite.
Also, according to the optical system of the present invention, it is preferable to constitute the catadioptric imaging system as an optical system which is telecentric on the first and second plane sides. Since the catadioptric imaging system is thus formed to be telecentric on the both sides, it is possible to suppress an image distortion due to displacement of the mask provided on the first plane or the wafer provided on the second plane in the direction of the optical axis into an insignificantly small amount. In addition, it is possible to realize an ideal both-side telecentric optical system in which the numerical apertures of the respective view fields are equal to each other, by disposing an aperture stop in the vicinity of the rear focal position of the first lens group.
Moreover, according to the present invention, it is preferable to satisfy the following conditional expression (2):
0.7 less than |xcex21/xcex22| less than 3.5xe2x80x83xe2x80x83(2). 
Here, xcex21 represents the magnification of the first imaging optical system, and xcex22 represents the magnification of the second imaging optical system.
The conditional expression (2) defines an appropriate range for the ratio xcex21/xcex22 between the magnification xcex21 of the first imaging optical system and the magnification xcex22 of the second imaging optical system. By satisfying the conditional expression (2), it is possible to compensate (cancel) the aberrations which are generated by the lens component of the first imaging optical system with the aberrations which are generated in the reflecting mirror, and the like, of the second imaging optical system in a good balance, whereby a high resolution of 0.1 xcexcm or lower can be realized. However, when the intermediate image is formed in the refracting optical member, this refracting optical member is assumed to belong to the first imaging optical system.
Above the upper limit of the conditional expression (2), the aberrations generated in the second imaging optical system become larger, and excellent correction particularly of a spherical aberration, a coma, and a chromatic aberration becomes unfavorably difficult. On the other hand, below the lower limit of the conditional expression (2), the aberrations generated in the first imaging optical system become larger, so that the entire system is inevitably and unfavorably enlarged. In order to further reduce the size of the optical system and to achieve the sufficient image forming performance, it is preferable to set the upper limit of the conditional expression (2) to be 2.5 and the lower limit to be 0.85. Also, when the optical system is applied to a projection optical system of a projection exposure apparatus, the magnification (xcex21xc3x97xcex22) of the entire system is set within a range from 0.12 to 0.33, whereby the size or the precision of the mask can be set within a practicable range.
A catadioptric imaging system according to a third aspect of the invention is provided with a first imaging optical system of a dioptric type for forming an intermediate image of the first plane and a second imaging optical system of a catadioptric type for forming a final image of said first plane on a second plane with reduction magnification on the basis of the radiation from said intermediate image. The first imaging optical system comprises a first lens group of a positive refracting power, an aperture stop, and a second lens group of a positive refracting power in this order from the first plane side. The second image optical system comprises a primary mirror which is constituted by a first reflecting surface of a concave form having a first radiation transmitting portion at the center thereof, a secondary mirror which is constituted by a second reflecting surface having a second radiation transmitting portion at the center thereof, and a refracting member which is separated from the first reflecting surface and the second reflecting surface. The radiation from the first imaging optical system passes through the first radiation transmitting portion of the primary mirror and the refracting member to be reflected by the second reflecting surface, the radiation reflected by the second reflecting surface passes through the refracting member to be reflected by the first reflecting surface, and the radiation reflected by the first reflecting surface passes through the refracting member and the second radiation transmitting portion of the secondary mirror to form the final image on the second plane.
In the catadioptric imaging system described above, the intermediate image (primary image) of the first plane is formed through the first and second lens groups of the first imaging optical system. Then, in the vicinity of the forming position of the intermediate image, there is provided the primary mirror having the first radiation transmitting portion (the central opening) at the center thereof and having the first reflecting surface with a concave form, so that the diameter of this central opening of the primary mirror is reduced to suppress the central shielding rate, thereby avoiding deterioration of the image forming performance. Further, such arrangement is employed in which in the vicinity of the second plane (the wafer plane, that is, the final image plane), there are provided the secondary mirror having the second radiation transmitting portion (the central opening) and the refracting member separated from the secondary mirror on its primary mirror side, so that the central opening of this secondary mirror can be approximated to the second plane, in which the secondary mirror is arranged as thin as possible in the direction of the optical axis. Thus, the central shielding rate is reduced to avoid deterioration of the image forming performance.
According to the catadioptric imaging system of the third aspect of the invention described above, even when a radiation having the wavelength of 180 nm or less such as an F2 laser beam is used, the size of the primary mirror size is not necessarily increased and the image-side NA and the image circle in the predetermined sizes can be securely obtained, so that it is possible to realize such a catadioptric imaging system as capable of obtaining a high resolution of, for example, 0.1 xcexcm or less by the use of a smaller number of lenses.
Also, according to the optical system of the third aspect of the invention, it is preferable that the refracting member has a negative refracting power, and the following conditional expression (3) is satisfied:
xe2x88x9285 less than f1/d1 less than xe2x88x9210xe2x80x83xe2x80x83(3). 
Here, f1 represents the focal length of the refracting member, and d1 a distance between the secondary mirror and the refracting member along the optical axis, respectively. The conditional expression (3) defines a condition for correcting a chromatic aberration satisfactorily. Above the upper limit of the conditional expression (3), a deviation in the curvature of field for each wavelength becomes larger, so that it becomes impossible to conduct excellent chromatic aberration correction. A high-order aberration, such as a chromatic coma, is also unfavorably generated. Conversely, below the lower limit of the conditional expression (3), the first-order chromatic aberration is not sufficiently corrected unfavorably. In this respect, if the lower limit of the conditional expression (3) is set at xe2x88x9275 and the upper limit is at xe2x88x9220, the chromatic aberration correction can be conducted in a wider wavelength range with more excellency.
Also, in the optical system of the third aspect of the invention, it is preferable that the refracting member has a refracting surface with the concave surface facing the second plane side. More preferably, the refracting member is in the form of a meniscus. In this case, on the refracting surface of the refracting member on the first plane side and on the refracting surface of the refracting member on the second plane side, the incident angle and the exit angle of a beam which is reflected by the primary mirror and then advances toward the second plane can be comparatively small, and when a beam having a large numerical aperture is guided onto the second plane, generation of a high-order aberration on these refracting surfaces can be suppressed. Also, of the above refracting member, it is preferable that the refracting surface on the second plane side has a larger negative refracting power than the refracting surface on the first plane side. In this case, the incident angle and the exit angle of a beam on and from a lens can be reduced, so as to prevent a high-order aberration from being generated.
Moreover, according to the optical system of the third aspect of the invention, it is preferable to satisfy the following conditional expression (4):
0.6 less than |xcex21/xcex22| less than 3.5xe2x80x83xe2x80x83(4). 
Here, xcex21 represents the magnification of the first imaging optical system, and xcex22 represents the magnification of the second imaging optical system, respectively. The conditional expression (4) defines an appropriate ratio between the magnification of the first imaging optical system and the magnification of the second imaging optical system. By satisfying the conditional expression (4), it is possible to compensate (cancel) an aberration which is generated by the lens component of the first imaging optical system and an aberration which is generated in the reflecting mirror, or the like, of the second imaging optical system in a good balance, whereby a high resolution of 0.1 xcexcm or less can be realized. However, when the intermediate image is formed in the refracting optical member, this refracting optical member is assumed to belong to the first imaging optical system.
Above the upper limit of the conditional expression (4), aberrations generated in the second imaging optical system become larger, and excellent correction of a spherical aberration, a coma, and a chromatic aberration, particularly, becomes unfavorably difficult. On the other hand, below the lower limit of the conditional expression (4), aberrations generated in the first imaging optical system become larger, so that the entire system is inevitably and unfavorably enlarged. In order to further reduce the size of the optical system and to achieve the sufficient image forming performance, it is preferable to set the upper limit of the conditional expression (4) at 2.5 and the lower limit at 0.85. Also, when the optical system is applied to a projection optical system of a projection exposure apparatus, the magnification (xcex21xc3x97xcex22) of the entire system is set within a range from 0.12 to 0.33, whereby the size or the precision of the mask can be set within a practicable range.
A catadioptric imaging system according to a fourth aspect of the invention is provided with a first imaging optical system for forming an intermediate image on a first plane and a second imaging optical system for forming a final image of the first plane on a second plane with reduction magnification on the basis of the radiation from said intermediate image. One of the first and second imaging optical systems comprises a primary mirror which is provided with a first reflecting surface with a concave form having a first radiation transmitting portion at the center thereof, and a secondary mirror which is provided with a second reflecting surface having a second radiation transmitting portion at the center thereof. The primary mirror and the secondary mirror are positioned such that a radiation directed to the first radiation transmitting portion is reflected by the second reflecting surface through the first radiation transmitting portion, a radiation reflected by the second reflecting portion is reflected by the first reflecting surface, and a radiation reflected by the first reflecting surface passes through the second radiation transmitting portion of the secondary mirror. The catadioptric imaging system comprises a central shielding member for shielding a radiation which is not reflected by the first and second reflecting surfaces and is directed to the second plane, and a variable aperture stop arranged to have a variable aperture size, in which the central shielding member and the variable aperture stop are disposed at positions different from each other in the direction of the optical axis of the catadioptric imaging system.
According to the optical system of the fourth aspect of the invention, it is possible to shield an unnecessary light with respect to the beams of all the image heights (the object heights) effectively by disposing the central shielding member in the vicinity of a paraxial pupil position (the position at which the paraxial chief ray crosses the optical axis). Then, it is possible to easily avoid a mechanical interference between the mechanical structure of the variable aperture stop and a mechanism for holding the central shielding member by disposing the variable aperture stop at a position different from that of the central shielding member in the direction of the optical axis so as to suppress practically sufficiently an influence of vignetting in all the image heights (the object heights) when the aperture size of the variable aperture stop changes, that is, a difference in the numerical apertures among the beams in all the image heights (the object heights).
Here, when the variable aperture stop and the central shielding member are disposed at the same position, not only that a mechanical interference is brought about between the variable aperture stop and the central shielding member, but also a difference in the numerical apertures is unfavorably generated due to a difference in the image height (the object-height) when the aperture size of the variable aperture stop is changed. In this case, in order to prevent a difference in the numerical apertures due to a difference in the image height (the object height), it can be considered to completely correct the field curvature of the pupil. However, in this case, the catadioptric imaging system becomes unfavorably complicated with, for example, the enlarged total length or the increased number of the lenses.
According to the optical system of the fourth aspect of the invention, the first imaging optical system comprises a first lens group of a positive refracting power, the variable aperture stop, and a second lens group of a positive refracting power in that order from the first plane side, the second imaging optical system comprises the primary mirror and the secondary mirror, and the central shielding member is disposed in the first imaging optical system.
Also according to the above optical system, an unnecessary light with respect to the beams of all the image heights supplied to the second imaging optical system including the primary mirror and the secondary mirror can be effectively intercepted in the first imaging optical system, whereby it is possible to easily avoid a mechanical interference between the mechanical structure of the variable aperture stop and the mechanism for holding the central shielding member and it is also possible to suppress an influence of vignetting in all the image heights (the object heights) practically sufficiently when the aperture size of the variable aperture stop is changed. Note that, in this arrangement, the variable aperture stop is preferably disposed between the central shielding member and the first lens group. In this manner, a difference in the numerical apertures when the aperture size of the variable aperture stop is changed can be substantially equal in all the image heights (the object heights).
A catadioptric imaging system according to a fifth aspect of the invention is provided with a first imaging optical system of a dioptric type for forming an intermediate image of a first plane and a second imaging optical system of a catadioptric type for forming a final image of said first plane on a second plane with reduction magnification on the basis of the radiation from said intermediate image. The first imaging optical system comprises a first lens group of a positive refracting power, an aperture stop, and a second lens group of a positive refracting power in this order from the first plane side. The second image optical system comprises a primary mirror which is provided with a first reflecting surface with a concave form having a first radiation transmitting portion at the center thereof, and a secondary mirror which is provided with a second reflecting surface having a second radiation transmitting portion at the center thereof. The radiation incident on the second imaging optical system passes through the first radiation transmitting portion of the primary mirror and is reflected by the second reflecting surface, the radiation reflected by the second reflecting surface is reflected by the first reflecting surface, and the radiation reflected by the first reflecting surface passes through the second radiation transmitting portion of the secondary mirror to form the final image on the second plane. A refracting member provided in the catadioptric imaging system is disposed only in a part of an optical path between the first plane and the second plane, excluding the portion between the first radiation transmitting portion and the second radiation transmitting portion.
In the optical system of the fifth aspect of the invention, the intermediate image is formed by the first imaging optical system, and in the vicinity of the forming position of the intermediate image, there is provided the primary mirror having the first radiation transmitting portion (the central opening) at the center thereof and having the first reflecting surface of the negative refracting power, so that the diameter of this central opening of the primary mirror is reduced, thereby avoiding deterioration of the image forming performance which may be caused by increase of the central shielding rate. Further, such arrangement is employed in which in the vicinity of the second plane (the image plane), there is provided the secondary mirror with the second reflecting surface having the second radiation transmitting portion (the central opening) at the center thereof, so as to be thin in the direction of the optical axis. Thus, it is possible to prevent deterioration of the image performance which may be caused by the increase of the central shielding rate, as described above. In this case, the refracting member is disposed only in a part, excluding the portion between the central opening on the primary mirror side and the central opening on the secondary mirror side. That is, the primary mirror and the secondary mirror no longer utilize the rear surface reflection of the refracting member, so that it is possible to easily and effectively prevent deterioration of the image forming performance which is caused by a change of the surface form of the refracting member due to absorption of a comparatively large amount of the illumination heat caused by the rear surface reflection. Also, it is no longer required to dispose the refracting member between the primary mirror and the secondary mirror, so that it is possible to easily prevent the refracting member from being enlarged with the primary mirror and the secondary mirror the diameters of which are inevitably enlarged for reducing the shielding rate. Further, it is also possible to easily avoid deterioration of the image forming performance including the reduction of contrast, generation of ghost; and the like, caused by a reflected light which can not transmit through the refracting surface of the refracting member disposed between the primary mirror and the secondary mirror and becomes a stray light.
According to the catadioptric imaging systems of the fifth aspect of the invention described above, even when a radiation having the wavelength of 180 nm or less such as an F2 laser beam is used, the primary mirror size is not necessarily increased and an image-side NA and an image circle in the predetermined sizes can be securely obtained, so that it is possible to realize a catadioptric imaging system which is capable of obtaining a high resolution of, for example, 0.1 xcexcm or less by using a smaller number of lenses.
The optical system of the fifth aspect of the invention is further provided a chromatic aberration correction lens which is disposed in a portion in an optical path between the intermediate image and the first reflecting surface except a part between the first radiation transmitting portion and the second radiation transmitting portion. In this optical system, a chromatic aberration can be corrected by the chromatic aberration correction lens having a small diameter without deterioration of the image forming performance.
Also, in the optical system of the fifth aspect of the invention, it is preferable to satisfy the following conditional expression (5):
xe2x88x921.10 less than f2/|d2| less than xe2x88x920.15xe2x80x83xe2x80x83(5). 
Here, f2 represents the focal length of the chromatic aberration correction negative lens disposed between the intermediate image and the first reflecting surface, and d2 a distance between the first reflecting surface and the second reflecting surface, respectively. Below the lower limit of the conditional expression (5), the power of the chromatic aberration correction negative lens becomes smaller so that it is difficult to realize excellent chromatic aberration correction. Conversely, above the upper limit of the conditional expression (5), another aberration may be generated so that it is difficult to produce the chromatic aberration correction negative lens itself. In this respect, if the upper limit of the conditional expression (5) is set at xe2x88x9225 and the lower limit at xe2x88x9270, the chromatic aberration can be corrected in the entire image field with excellency without increasing the size of the optical system.
Also, according to the optical system of the fifth aspect of the invention, it is possible to dispose a central shielding member in the vicinity of the rear focal position of the first lens group for intercepting a radiation incident on the center of the second imaging optical system.
In the above optical system, it is possible to effectively intercept the unnecessary radiation with respect to the beams of all the image heights supplied to the second imaging optical system including the primary mirror and the secondary mirror, by means of the central shielding member disposed in the first imaging optical system.
Also, in the optical system of the fifth aspect of the invention, the first lens group may have the field curvature of the pupil, and the central shielding member and the aperture stop may be disposed at positions which are separated from each other in the direction of the optical axis of the first imaging optical system.
In the above optical system, it is possible to arrange such that a mechanical interference between the mechanical structure of the variable aperture stop and the mechanism for holding the central shielding member can be easily avoided. Further, it is possible to obtain a uniform image forming performance in the entire image field by disposing the central shielding member at the rear focal position of the first lens group and the aperture stop at a position separated from this rear focal position by the field curvature of the pupil. That is, it is possible to obtain the same area and position of the shielded portion with respect to any angle of view. Even when the aperture size of the variable aperture stop is changed, the influence of vignetting caused by the central shielding member in all the image heights can be sufficiently reduced.
Also, according to the optical system of the fifth aspect of the invention, it is possible to make both the first plane side and the second plane side telecentric optical systems.
In the above optical system, it is possible to reduce an influence of an image distortion which is caused by a slight deviation in the direction of the optical axis of the first plane and the second plane such as a movement or a bent to the extent which can be ignored.
Also, the optical system of the fifth aspect of the invention has ten or more refracting surfaces, at least five of which may have aspherical forms.
In the above optical system, it is possible to obtain a sufficient brightness with a small amount of loss with the reduced number of the refracting lenses, and to have little fluctuation in the aberrations due to heat generation. That is, when an exposure wavelength becomes 180 nm or less, the film materials used for an antireflection coat which is applied on the surfaces of the refracting lenses are limited, so as not to obtain a sufficient performance as the antireflection coat. For this reason, it is required to minimize a transmitting surface. Thus, it is possible to extremely reduce the number of the refracting lenses while realizing a resolution of 0.1 xcexcm or less, by forming at least five of the ten or more refracting surf aces to be aspherical, as described above.
In the optical system of the fifth aspect of the invention, it is also preferable to satisfy the following conditional expressions (6) and (7).
0.15 less than |xcex2/xcex23| less than 0.95xe2x80x83xe2x80x83(6). 
0.10 less than |xcex2/xcex24| less than 0.50xe2x80x83xe2x80x83(7). 
Here, xcex2 represents the magnification of the entire catadioptric imaging system mentioned above, xcex23 represents the magnification of the first reflecting surface, and xcex24 represents the magnification of the second reflecting surface.
Below the lower limits of the conditional expressions (6) and (7), a ratio of bearing the magnification in the first imaging optical system increases, so that the diameter of the lens for constituting the first imaging optical system increases. On the other hand, above the upper limits the conditional expressions (6) and (7), the intermediate image becomes larger; so that the central shielding may become larger or an operational distance on the second plane side may not be sufficiently secured. As a result, an excellent image forming performance can not be obtained. Note that if the upper limit of the conditional expression (6) is set at 0.8 and the lower limit at 0.3, and the upper limit of the conditional expression (7) is set at 0.28 and the lower limit at 0.28, it is possible to realize an optical system having a more excellent image forming performance and a sufficiently long operational distance.
Also, in the optical system of the fifth aspect of the invention, the refracting member for constituting the catadioptric imaging system can be disposed only in an optical path between the first plane and the first radiation transmitting portion.
In the above optical system, since there is no refracting member in an optical path from the first reflecting surface to the second plane, it is no longer required to dispose a refracting member between the primary mirror and the secondary mirror, and further, between the secondary mirror and the second plane. As a result, it is possible to prevent more securely the image forming performance from being deteriorated due to the radiation which is reflected by the refracting surface of the refracting member and becomes a stray light.
The optical system of the fifth aspect of the invention preferably satisfies the following conditional expression (8):
0.04 less than |d3/d2| less than 0.08xe2x80x83xe2x80x83(8). 
Here, d2 represents a distance between the first reflecting surface and the second reflecting surface, and d3 a distance between the second reflecting surface and the second plane.
Below the lower limit of the conditional expression (8), it becomes impossible to obtain a sufficiently large operational distance on the second plane side without enlarging the secondary mirror. On the other hand, above the upper limit of the conditional expression (8), the central shielding becomes large so that an optical system having a sufficiently excellent image forming performance can not be achieved. In this respect, it is preferable to change the upper limit of the conditional expression (8) from 0.065 to xe2x88x920.3, and to set the lower limit of the conditional expression (8) at 0.043.
According to the optical system of the fifth aspect of the invention, it is arranged such that the second imaging optical system forms the intermediate image which is formed by the first imaging optical system as the final image mentioned above on the second plane.
According to the optical system of the fifth aspect of the invention, the first radiation transmitting portion of the primary mirror and the second radiation transmitting portion of the secondary mirror may be disposed at positions containing the optical axis of the catadioptric imaging system.
A catadioptric imaging system according to a sixth aspect of the invention is provided with a first imaging optical system of a dioptric type for forming an intermediate image of a first plane and a second imaging optical system for forming a reduced image of the first plane on a second plane on the basis of the radiation from said intermediate image. The first imaging optical system comprises an aperture stop, a first lens group disposed between the aperture stop and the first plane, and a second lens group disposed between the aperture stop and the intermediate image. The second image optical system comprises a primary mirror which is provided with a first reflecting surface having a first radiation transmitting portion at the center thereof, and a secondary mirror which is provided with a second reflecting surface having a second radiation transmitting portion at the center thereof. The first reflecting surface is a front surface reflecting surface with a concave form, and the second reflecting surface has a negative power. The second imaging optical system is arranged such that a radiation incident on the second optical system passes through the first radiation transmitting portion of the primary mirror and is reflected by the second reflecting surface of the secondary mirror, the radiation reflected by the second reflecting surface is reflected by the first reflecting surface, and the radiation reflected by the first reflecting surface passes through the second radiation transmitting portion of the secondary mirror to reach the second plane.
In the optical system of the sixth aspect of the invention, the primary mirror is disposed in the vicinity of the forming position of the intermediate image, so that the size of the first radiation transmitting portion disposed in the central part of this primary mirror is reduced and the central shielding rate is increased, thereby avoiding deterioration of the image forming performance. Further, such arrangement is employed in which the secondary mirror is disposed in the vicinity of the second plane, to be thin in the direction of the optical axis. Thus, it is possible to prevent deterioration of the image performance which may be caused by the increase of the central shielding rate, in the same manner as described above. In this case, the first reflecting surface on the primary mirror side is the front surface reflecting surface of the negative power, which does not utilize the rear surface reflection of a refracting member, so that it is possible to easily and effectively prevent deterioration of the image forming performance which may be caused by absorption of the illumination heat generated by the rear surface reflection in the refracting member. Also, it is no longer required to prepare a large-sized refracting member because of the primary mirror which inevitably has a comparatively large diameter in order to reduce the shielding rate. Further, it is also possible to easily avoid the deterioration of the image forming performance including a reduction of contrast, generation of ghost, or the like, caused by a radiation which can not transmit through the refracting surface of the refracting member, but is reflected thereby, and becomes a stray light, like in case in which the primary mirror is used as a rear surface reflecting mirror.
According to the catadioptric imaging systems of the sixth aspect of the invention described above, even when a radiation having the wavelength of 180 nm or less such as an F2 laser beam is used, the primary mirror size is not necessarily increased and an image-side NA and an image circle in the predetermined sizes can be securely obtained, so that it is possible to realize a catadioptric imaging system which is capable of attaining a high resolution of, for example, 0.1 xcexcm or less by using a smaller number of lenses.
According to the optical system of the sixth aspect of the invention, the second imaging optical system may have a refracting member which is disposed in an optical path between the first reflecting surface and the second reflecting surface.
In the above optical system, various kinds of aberrations can be suppressed by, for example, correcting the chromatic aberration of the first imaging optical system by using the refracting member, so as to further enhance the image forming performance.
Also, in the optical system of the sixth aspect of the invention, one of the optical surfaces of the refracting member in the second imaging optical system is provided with a lens surface having a negative refracting power, while the other with the second reflecting surface.
In the above optical system, it is possible to easily ensure a sufficient operational distance (working distance) on the image side while approximating the second plane on which a reduced image is formed to the second radiation transmitting portion of the secondary mirror.
Also, according to the optical system of the sixth aspect of the invention, the optical surface on the secondary mirror side of the refracting member in the second imaging optical system may be arranged to be separated from the second reflecting surface of the secondary mirror.
In this case, it is no longer required to utilize the rear surface reflection in the second imaging optical system, so that it is possible to correct the chromatic aberration, or the like, by means of the refracting member while decreasing absorption of the thermal energy, thereby enhancing the image forming performance.
In the optical system of the sixth aspect of the invention, the refracting member for constituting the catadioptric imaging system is disposed only in a portion of an optical path between the first plane and the second plane, excluding a part between the first radiation transmitting portion and the second radiation transmitting portion.
In this case, the primary mirror and the secondary mirror no longer utilize the rear surface reflection of the refracting member, so that it is possible to easily and effectively prevent deterioration of the image forming performance which is caused by a change of the surface form of the refracting member due to absorption of a comparatively large amount of illumination heat by the rear surface reflection. Also, it is no longer required to dispose a refracting member between the primary mirror and the secondary mirror, so that it is possible to easily prevent the refracting member from being enlarged with the increase of the sizes of the primary mirror and the secondary mirror the diameters of which inevitably become comparatively large in order to reduce the shielding rate. It is further possible to easily prevent the deterioration of the image forming performance including reduction of contrast, generation of ghost, or the like, caused by a reflected radiation which can not pass through the refracting surface of the refracting member disposed between the primary mirror and the secondary mirror, but is reflected thereby to become a stray light.
An optical system according to the seventh aspect of the invention is an optical system which is provided with an imaging optical system for optically conjugate a first plane and a second plane to each other, and a substrate position detecting system for photoelectrically detecting the position of a substrate with respect to the second plane. The imaging optical system comprises a primary mirror consisting of a first reflecting surface with a concave form having a first radiation transmitting portion at the center thereof, and a secondary mirror consisting of a second reflecting surface disposed between the primary mirror and the second plane and having a second radiation transmitting portion at the center thereof, and a base member provided with the second reflecting surface formed on the surface thereof. The substrate position detecting system is provided with a light guiding system for guiding a detection light, after passing it through the base member of the secondary mirror, to a detection area which is formed by projecting the second radiation transmitting portion onto the second plane, and a light receiving system for guiding the reflected light in the detection area, after passing it through the base member of the secondary mirror, to a photoelectric conversion unit.
In the optical apparatus of the seventh aspect of the invention, it is required to position the secondary mirror in the vicinity of the second plane in order to reduce the central shielding rate. However, since being a surface reflecting mirror, the secondary mirror has the thickness in the direction of the optical axis, and a substantial operational distance on the image side, that is, a space between the second plane and the base member of the secondary mirror is required to be small, compared with that in a conventional projection optical system. For this reason, in case of detecting the position of the second plane, it is difficult to conduct the focal detection by the conventional oblique incidence method. Examples of such optical apparatus for conducting the focal detection by the oblique incidence method as described above are disclosed in Japanese Patent Application Laid-Open No. HEI 6-66543, Japanese Patent Application Laid-Open No. HEI 8-219718, Japanese Patent Application Laid-Open No. HEI 9-304016, and Japanese Patent Application Laid-Open No. HEI 10-82611. In the structure of the present invention, it is preferable that the light guiding system of the substrate position detecting system is arranged to guide a detection light from a side surface of the base member of the secondary mirror, and to guide the detection light emitted from the side surface of the base member to a detection area. The light receiving system of the substrate position detecting system is preferably arranged to guide a light which is reflected in the detection area into the base of the base member of the secondary mirror member through the side surface thereof, and to guide the light emitted from the side surface of the base member to the photoelectric conversion unit. With this structure, it is preferable that a light advancing through the base member of the secondary mirror is guided to be reflected on the surface on the first plane side and the surface on the second plane side of the secondary mirror. Also with this structure, the side surface of the base member through which the detection light and the reflected light in the detection area pass is formed to be planar. With this structure of the optical apparatus, it becomes possible to conduct a focusing operation with high precision.
A projection exposure apparatus in which the optical apparatus of the seventh aspect of the invention is incorporated is provided with an illumination optical system for illuminating a mask with a predetermined pattern formed thereon, and the above-mentioned optical apparatus for projecting an image of the predetermined pattern of the mask disposed on the first plane onto a photosensitive substrate which is disposed on the second plane. According to this projection exposure apparatus, it becomes possible to conduct exposure with high precision by conducting a focusing operation with high precision.
Also, an exposure method using the optical apparatus of the seventh aspect of the invention comprises the step of illuminating a mask on which a predetermined pattern is formed by the illumination radiation, and the step of projecting an image of the predetermined pattern of the mask disposed on the first plane onto a photosensitive substrate which is disposed on the second plane.
According to this exposure method, it becomes possible to conduct exposure with high precision by conducting a focusing operation with high precision.
In the catadioptric imaging system according to one of the first to fifth aspects of the invention described above, all of the refracting optical members for constituting the catadioptric imaging system may be formed of fluorite, whereby it becomes possible to use an exposure light having the wavelength of 180 nm or less such as an F2 laser beam(157 nm), so as to realize a high resolution of 0.1 xcexcm or less.
Also, according to the catadioptric imaging system of one of the first to fifth aspects of the invention, in an optical path between the first lens group and the second lens group in the first imaging optical system, a light shielding member may be disposed for intercepting some of beams centering around the optical axis, out of incident beams. In the optical system of one of the first and fifth aspects of the invention, there are provided two reflecting surfaces (the reflecting surface of the primary mirror and the reflecting surface of the secondary mirror) each having an opening at the center thereof, so that a beam advancing along the optical axis from the first plane is not reflected by any of these reflecting surfaces and becomes a stray light directly reaching the second plane through the central openings. Then, the above-mentioned stray light can be removed by disposing the shielding member for intercepting some of the beams centering around the optical axis, out of the incident beams, in the optical path between the first lens group and the second lens group of the first imaging optical system (for example, in the vicinity of the aperture stop). With this structure, it is also possible to make the central shielding portions of the beams to be the same in the respective fields of view so as to effectively avoid the image forming performance from changing in the fields of view.
Also, according to the catadioptric imaging system of one of the first to fifth aspects of the invention, it is possible to further provide a filed aperture disposed in the vicinity of the forming position of the primary image. Thereby, it is also possible to prevent an unnecessary light directed to an area other than an exposure area from reaching the second plane, and to prevent a stray light such as flare generated in the first imaging optical system from reaching the second plane.
Also, in the catadioptric imaging systems according to one of the first to fifth aspects of the invention, the catadioptric imaging system described above may have an image circle having the diameter of 10 mm or larger on the second plane, whereby when it is applied to the projection optical system of the projection exposure apparatus, a batch exposure can be conducted for a large exposure area, which results in enhancement of the throughput. Note that, in the present invention, the image circle of the catadioptric imaging system indicates an area in which aberrations have been corrected on the image plane of this catadioptric imaging system.
In the catadioptric imaging system according to one of the first to fifth aspects of the invention, the primary mirror in the second imaging optical system may be formed of a material having a coefficient of linear expansion of 3 ppm/xc2x0 C. or less. In the optical system according to one of the first to fifth aspects of the invention, the clear aperture diameter of the primary mirror becomes large and the refracting power thereof also becomes large, so that a plane change of the reflecting surface of the primary mirror (specifically, the front surface reflecting surface of the optical system according to the first or second aspect of the invention, or the first reflecting surface according to the third or fourth aspect of the invention) due to the illumination heat of the exposure light affects the image forming performance greatly. Then, it is possible to prevent the deterioration of the image forming performance during the exposure caused by the plane change of the reflecting surface of the primary mirror by forming a base for supporting the reflecting surface of the primary mirror of a material having a coefficient of linear expansion of 3 ppm/xc2x0 C. or less. For example, a material called ULE (trade name) put on the market by Corning, Inc., may be used as such material. This ULE (Ultra Low Expansion Titanium Silicate Glass) has a coefficient of linear expansion of xcex1=5xc3x9710xe2x88x928/xc2x0 C.=0.05 ppm/xc2x0 C. (see Applied Optics, Vol. 24, p3330 (1985); Vol. 23, p2852, p3014 (1984)).
Also, in the catadioptric imaging system according to one of the first to fifth aspects of the invention, in the vicinity of the rear focal position of the first lens group in the first imaging optical system, an optical element may be disposed for relatively giving at least one of an intensity difference, a phase difference, and a difference in polarized state to a beam which passes through a first area within a section of the beam and a beam which passes through a second area different from the first area in the section of the beam. In this manner, it is possible to obtain a greater depth of focus for the catadioptric imaging system. In this case, an optical element for giving an intensity difference to a beam (a light shielding filter) intercepts or attenuates the central beam while transmitting surrounding beams. Also, an optical element for giving a phase difference to a beam (a phase filter) makes a phase difference between the central beam and the surrounding beams. Further, an optical element for giving a difference in polarized state to a beam (a polarizing filter) makes the direction of polarization of the central beam and that of the surrounding beams to be crossing each other. In this respect, examples in which the light shielding filter is disposed at the pupil position of a projection optical system of a projection exposure apparatus are disclosed in Japanese Patent Application Laid-Open No. HEI 5-234846 and No. HEI 5-234847. Also, examples in which the phase filter is disposed at the pupil position of a projection optical system of a projection exposure apparatus are disclosed in Japanese Patent Application Laid-Open No. HEI 6-215999 and No. HEI 6-244082. Further, examples in which the polarizing filter is disposed at the pupil position of a projection optical system of a projection exposure apparatus are disclosed in Japanese Patent Application Laid-Open No. HEI 6-120110.
Also, in the catadioptric imaging system according to either of the first to fifth aspects of the invention, all of the refracting optical members, the primary mirror and the secondary mirror for constituting the above catadioptric imaging system may be disposed along the single optical axis. In this manner, it becomes possible to design and produce a lens barrel by a technology derived from the conventional art of producing a circular cylinder barrel of dioptric type. As a result, it is possible to achieve the production of higher precision without difficulties.
Also, in the catadioptric imaging system according to either of the first to fifth aspects of the invention, the number of the lenses for constituting this catadioptric imaging system may be ten or less. Thereby, when an extremely fine pattern is projected on a photosensitive substrate by using an exposure radiation having the wavelength of 180 nm or less, it is possible to suppress reduction of transmittance, to thereby reduce a radiation amount loss.
Also, in the catadioptric imaging system according to one of the first to fifth aspects of the invention, the first imaging optical system comprises a central shielding member which is disposed at a position different from the aperture stop in the direction of the optical axis for intercepting light in the vicinity of the optical axis. Thereby, the above-mentioned stray light advancing straight along the optical axis can be removed.
A projection exposure apparatus incorporating the catadioptric imaging system according to one of the first to fifth aspects of the invention therein is provided with an illumination optical system for illuminating a mask on which a predetermined pattern is formed by an illumination radiation in the ultraviolet region, and the above-mentioned catadioptric imaging system for projecting the image of the predetermined pattern of the mask disposed on the first plane onto a photosensitive substrate. According to this projection exposure apparatus, it becomes possible to project an extremely fine pattern onto the photosensitive substrate by using an exposure radiation having the wavelength of, for example, 180 nm or less, so as to achieve the exposure with high precision. Also, since an F2 laser which has been subjected to a comparatively simple band narrowing process can be used as an exposure light source, a large exposure power can be obtained. Further, since the maintenance cost of the laser beam source becomes lower, it is possible to realize a projection exposure apparatus with reduced cost of the laser beam source and high productivity.
The projection exposure apparatus further comprises a first stage for supporting the mask to be movable along a predetermined scanning direction, and a second stage for supporting the photosensitive substrate to be movable along the predetermined scanning direction, so as to conduct the exposure by moving the first and second stages with respect to the catadioptric imaging system. With this projection exposure apparatus, a scanning type exposure becomes possible which moves the mask and the photosensitive substrate in synchronism with respect to the catadioptric imaging system.
The projection exposure apparatus may be arranged such that the first and the second stages are moved in the same direction when the exposure is conducted.
An exposure method incorporating the catadioptric imaging system according to one of the first to fifth aspects of the invention therein comprises the step of generating an illumination radiation in the ultraviolet range, the step of illuminating a mask on which a predetermined pattern is formed by the illumination radiation, and the step of projecting the image of the predetermined pattern of the mask disposed on the first plane onto a photosensitive substrate disposed on the second plane by using the above-mentioned catadioptric imaging system. According to this exposure method, it becomes possible to project an extremely fine pattern onto a photosensitive substrate by using an exposure light having the wavelength of, for example, 180 nm or less, so as to achieve the exposure with high precision. Also, since the F2 laser which has been subjected to the comparatively simple process of band narrowing can be used as the exposure light source, a large exposure power can be obtained.
By the above exposure method, it is possible to conduct an exposure while moving the mask and the photosensitive substrate with respect to the catadioptric imaging system. By this exposure method, it becomes possible to conduct an exposure of scanning type in which the mask and photosensitive substrate are moved in synchronism with respect to the catadioptric imaging system.
The above projection exposure apparatus may be arranged such that the exposure is conducted by moving the mask and the photosensitive substrate in the same direction with respect to the catadioptric imaging system.
A method of manufacturing a device by using the above projection exposure apparatus comprises the step of preparing the photosensitive substrate by applying a photosensitive material on the substrate, the step of forming the final image of the mask on the photosensitive substrate through the catadioptric imaging system, the step of developing the photosensitive material on the substrate, and the step of forming a pattern corresponding to the developed photosensitive material on the substrate. According to this device manufacturing method, it is possible to provide an electric device of high density and high precision comprising an extremely fine pattern.